A power transmission chain transmits power from a driving sprocket to one or more driven sprockets by forming an endless loop that wraps and engages the teeth of the sprockets. Rotation of the driving sprocket moves the chain thereby transmitting power through the chain to rotate the driven sprockets engaged by the chain. Power transmission chains are widely used in the automotive industry. In an engine timing application, a chain transmits power from at least one driving sprocket positioned on a crankshaft to at least one driven sprocket positioned on a camshaft. Other automotive applications of power transmission chains include transmitting power from a torque converter to a transmission and transmitting power in the transfer case of a four wheel drive vehicle. Power transmission chains are also widely used in industrial applications.
One type of power transmission chain is known as a “silent chain.” A typical silent chain comprises an endless loop formed by a series of links that are adjacent to each other along the chain and that are rotatably joined to adjacent links. Each link extends a distance in a chain direction between locations at which it is rotatably joined to adjacent links and conventionally forms two teeth that are adjacent to each other along the chain direction of the link. The teeth of a link are formed to engage the teeth of a sprocket and extend in a front direction that is perpendicular to the chain direction. Links of a silent chain also conventionally form two apertures, one near each end of the link along the chain direction. The apertures extend through the link in a lateral direction that is perpendicular to the chain direction and perpendicular to the front direction of the link. The links of a silent chain are typically formed by a row of substantially identical flat links, each forming teeth and apertures as described, that are positioned laterally adjacent to each other to collectively form a link of the chain.
A silent chain is formed by positioning rows of flat links adjacent to each other and partially overlapping along the chain direction so that apertures at adjacent ends of links are aligned. Pivots, such as pins, extend through the aligned apertures to rotatably join adjacent links. Rows of links are joined in this manner to form an endless loop in which the front direction of the links, the direction that the teeth extend from the chain links, is directed toward the region within the endless loop. Links having teeth extending into the region within the loop are referred to as inverted tooth links. The surface of the chain, along which the teeth extend, is referred to as the front side of the chain. The surface of the chain opposite the front side, facing outwardly from the region within the endless loop, is the back side of the chain.
The inverted tooth links (sometimes referred to as driving links) transfer power between the chain and a sprocket along the chain direction. Each tooth of a link defines an inside flank that faces generally along the chain direction toward the adjacent tooth of the link, and an outside flank that faces away from the inside flank of the tooth. The inside flanks of the teeth of an inverted tooth link meet at a crotch between the teeth. The teeth of a link may contact sprocket teeth along their inside flanks, along their outside flanks, or along both flanks. The contact between a link tooth flank and a sprocket tooth may transfer power or may be an incidental contact. Teeth of an inverted tooth link conventionally contact a sprocket on a flank of a sprocket tooth or at a root between adjacent sprocket teeth.
Inverted tooth links are positioned on a sprocket by contact with three sprocket teeth, one at each end of the link along the chain direction, and one between the teeth of the link. A row of links is positioned on a sprocket by contact of the teeth of the links comprising the row with sprocket teeth, by contact by outside flanks of teeth of the links of the adjacent rows, or by both. Contact with the sprocket at three locations along the chain direction limits motion of the row of links along the teeth of the sprocket.
Silent chains often include guide links. Guide links are conventionally flat plates that are positioned on the lateral outside edges of alternate rows of inverted tooth links. The guide links do not form teeth and are generally adjacent to the row of inverted tooth links. The guide links extend adjacent to the region between teeth of the inverted tooth links. The guide links on opposite lateral sides of a row are separated by approximately the lateral width of the sprocket teeth that extend between the teeth of the row of links. The guide links thereby act to position the chain laterally on a sprocket (i.e., maintain the chain on the center of the sprocket) but do not engage a sprocket between teeth of the sprocket. Guide links also increase the strength and stiffness of the chain.
Conventionally, the guide links on opposite lateral sides of a silent chain are aligned. Chains having guide links that are laterally aligned are generally stiffer and stronger at locations along the chain where guide links are located than at locations that do not have guide links. The chain must be designed to assure that the less strong locations without guide links are sufficiently strong. The cross-sections with guide links are stronger than the sections not having guide links and are therefore stronger than necessary. The cross sections with guide links have more material than is necessary, making the chain heavier than necessary.
A conventional silent chain drive is comprised of an endless loop silent chain that wraps at least two sprockets. Each sprocket is mounted to a shaft. Rotation of the shaft on which the driving sprocket is mounted transmits power from the driving sprocket through the chain to rotate a driven sprocket and the shaft to which the driven sprocket is mounted. FIG. 1 illustrates such a basic arrangement. A chain 3 forms an endless loop and partially wraps driving sprocket 1 and driven sprocket 2 that are within the loop. A front side 4 of the chain 3 is adjacent to the region within the endless loop formed by the chain 3. Teeth extend inwardly along the front side 4 to engage teeth of both the driving sprocket 1 and the driven sprocket 2. The back side 5 of the chain 3 is the side of the chain opposite the front side 4 and faces outwardly from the chain loop. Both the driving sprocket 1 and the driven sprocket 2 rotate in the same direction, shown counter-clockwise by FIG. 1.
Silent chains may also drive sprockets that engage the back side of the chain. Examples of devices that are driven by the back side of a silent chain (back-driven) include water pumps, injector pumps, and countershafts. FIG. 1 also shows a sprocket 6 that is back-driven by the chain 3. The driven sprocket 2 engages the teeth on the front side 4 of the chain 3 and rotates in the same direction as the driving sprocket 1, counter-clockwise in FIG. 1. Teeth of the back-driven sprocket 6 engage the back side 5 of the chain 3. The back-driven sprocket 6 rotates in the opposite direction of the driven sprocket 2.
Often, the back side of the links of a chain that engages and drives a sprocket is configured to engage a sprocket as a single tooth between two adjacent sprocket teeth. The teeth of sprockets engaged by the back side of such links are spaced apart by the length of the chain link. Consequently, these sprockets engage a chain link in fewer and farther separated locations than do front driven sprockets. This can result in a lower capacity for power transmission by back-driven sprockets. The lower power transmission capacity is acceptable in some applications because back-driven sprockets are frequently located in the slack region of the chain and/or are not required to transmit forces that are as large as can be transmitted by the front side of the silent chain.
Silent chain drives create noise at a variety of sources. One significant source of noise is the impact of sprocket teeth on chain teeth at the onset of engagement of the sprocket by the chain teeth. Among the factors that affect the level of the noise created by this impact are the velocity of impact between the chain and the sprocket and the mass of chain links contacting the sprocket. Noise created by engaging impact in silent chain drives is generally periodic with a frequency generally corresponding to the frequency of the chain teeth engaging sprocket teeth. This frequency is related to the number of teeth on the sprocket and the speed of the sprocket. The impacts can produce sound having objectionable pure sonic tones.
Chordal motion of a chain is another source of noise in power transmission chain drives, including silent chain drives, that is associated with engagement of a chain and sprocket. Chordal motion occurs as a chain link initially engages and begins to move with a sprocket. The sprocket can cause a movement of the span of free chain that is approaching the sprocket along the front to back direction of the chain. This vibratory movement, known as chordal fall, can also produce an objectionable pure sonic tone at the frequency of the chain mesh frequency or a multiple of it.
FIG. 2 illustrates chordal fall. As described above, chains are comprised of a series of pivotally joined rigid links. Consequently, chains are not continuously flexible and do not wrap a sprocket as a circular arc. Rather, a chain wraps around a sprocket in a series of line segments, or chords. As a silent chain engages a sprocket, the individual link teeth contact the surfaces of a sprocket tooth and extend between adjacent teeth of the sprocket. This contact forces the links to rotate around pivot joints with respect to adjacent links. The chain thus bends at the pivot joints between adjacent links as it engages and wraps around a sprocket.
FIG. 2 shows a chord C, which joins the pivot points A and B of a chain link at the position at which the point A engages a sprocket from a free chain run along a direction F. The chord C′ joins the pivot points at positions A′ and B′ where the point A has moved with the sprocket half way to the point B location. As shown, by moving from the location A to the location A′ the point A has moved a distance R in the direction T that is perpendicular to the direction F along the free span of the chain. The distance R is the chordal fall of the chain.
Because a chain repeatedly engages teeth, as described above, the change in position of the chain between repeated engagement of the sprocket causes is chordal fall. The chordal fall is proportional to the length of C between pivots and inversely proportional to the radius of the circular path traveled by the points A and B around the center of a sprocket. The longer that the distance between points at which the chain pivots to engage the sprocket, the chain pitch, the greater chordal fall will be. The smaller the radius of the circular path traveled by the points A and B around the center of a sprocket, the greater the chordal fall will be. For example, a chain with a long pitch engaging a sprocket with relatively small diameter will experience a pronounced amount of chordal fall. In contrast, for an ideal toothless belt engaging a smooth pulley continuously, the length of chord C will be infinitely small, and consequently the belt will experience no chordal fall.
Back-drive chain and sprocket engagement can exacerbate the problem of chordal fall by permitting greater uncontrolled chordal fall movement of a chain on a sprocket than occurs for front drive engagement. A tooth of a front-driven sprocket extends between teeth of a link on the front side of the chain, and two adjacent sprocket teeth engage the link at its ends along the chain direction, one adjacent to each tooth outer flank of the inverted tooth link. A tooth of a back-driven sprocket can only engage the back side of the chain between links along the chain direction. Because the back driven sprocket engages the chain at fewer and more widely separated locations than the front driven sprocket, the chordal fall motion increases due to inability of the chain and sprocket contact to control the chordal fall motion.
One method for decreasing chordal fall motion at back-driven sprockets is to use chains formed by interleaved flat links, that is a flat link is laterally adjacent to ends of two links that are aligned with and adjacent to each other along the chain direction. Sprocket teeth can extend into the back side of this chain at a distance that is only half the length of the link by alternately extending between links of laterally adjacent rows of teeth. The sprocket teeth must be in a series extending around the periphery of the sprocket and the teeth of adjacent series must be offset with respect to each other along the periphery of the sprocket. FIG. 3 shows a back drive sprocket with offset series of teeth, i.e. the teeth are not laterally aligned. In this arrangement, sprocket teeth only extend into the back side of the chain between links. However, the distance along the chain direction at which the chain successively engages the back drive sprocket is about half the length of a chain link. Consequently, the back-driven sprocket engages chain at three locations along the chain direction for each row of links.
The three offset series of teeth on the sprocket, as shown in FIG. 3, however, increase the expense of manufacturing the sprocket. When back-driven sprockets are not required to transmit high loads, moreover, the additional expense of the offset series of sprocket teeth is not required to assure adequate power transmission. Therefore, it is desirable to provide a more cost-effective way of limiting chordal fall motion in low load applications.
Thus, a need remains for a power transmission chain having a more uniform stiffness along its length then has been known. Further, a need also exists for a power transmission chain that engages a sprocket at both the front and back sides while maintaining control of chordal fall motion without requiring expensive and difficult to manufacture sprockets to engage the backside of the chain.